At present, a lead battery is installed in many vehicles. This lead batter supplies power to a starter motor, or many kinds of electric devices. The lead battery is inexpensive, but has the characteristics of a short cycle life, compared with a nickel hydride storage battery or a lithium ion storage battery. In the vehicles having the idle stop function (idle reduction function), as the number of charging and discharging is large, especially the life of the lead storage battery becomes short.
Then, a configuration in which the lead storage battery, and the nickel hydride storage battery or the lithium ion storage battery are connected in parallel is proposed. In this parallel circuit, it is proposed that a switch between the lead storage battery and the nickel hydride storage battery or the lithium ion storage battery is provided (for example, refer to patent literature 1). Mainly, this switch is turned off when power is supplied from the lead storage battery to the starter motor, and is used in order to stabilize a voltage supplied from the nickel hydride storage battery or the lithium ion storage battery to the electric devices. Further, it is proposed that a switch which controls charging or discharging of the nickel hydride storage battery or the lithium ion storage battery, is also provided (for example, refer to patent literature 1).